1. Field of the Invention
This invention relates to electrophotography, and more particularly to the technique of forming a latent image on a recording member by the use of a photosensitive screen having a multitude of tiny openings.
2. Description of the Prior Art
As the typical conventional electrophotography, there have been proposed a direct process such as, for example, electrofax, and an indirect process such as xerography. In the direct electrophotographic process, use is made of a specially treated image recording member coated with a photoconductive material such as zinc oxide. The direct method, however, has a drawback in that as the image formed on the recording member lacks in brightness, contrasts in the tones of the reproduced image are poor. Moreover, owing to a particular treatment rendered on the recording member, it is heavier than the conventional paper, hence a particular feeding means which is different from that for ordinary paper should be employed. According to the indirect process, an image of high contrast and good quality can be obtained by using ordinary paper as the image recording member. However, in the direct process, when a toner image is transferred to the recording member, the latter inevitably contacts the surface of the photosensitive member and further, cleaning means vigorously touches the surface of the photosensitive member for removal of the residual toner thereon with the consequence that the photosensitive member is impaired every time the transfer and cleaning operations are carried out. As the result of this, life of the expensive photosensitive member becomes shortened, which unavoidably means high cost in the image reproduction.
Therefore, in order to eliminate such drawbacks inherent in the conventional electrophotographic processes, there have been contemplated various methods such as, for example, those taught in the U.S. Pat. No 3,220,324, No. 3,645,614, No. 3,647,291, No. 3,680,954, and No. 3,713,734. In these patents, there is used a photosensitive member of screen type or grid type having a number of openings in the form of fine net. The electrostatic latent image is formed on the recording member by modulating flow path of ions through the screen or grid, after which the latent image formed on the recording material is visualized. In this case, the screen or grid which corresponds to the photosensitive member need be neither developed nor cleaned, hence the life of the screen or grid can be prolonged.
U.S. Pat. No. 3,220,324 teaches use of a conductive screen coated with a photoconductive material, through which an image exposure is effected onto the recording member simultaneously with the corona discharge. The flow of corona ions produced as the consequence of the corona discharge is modulated by the screen, whereby an electrostatic latent image is formed on the recording member. U.S. Pat. No. 3,680,954 teaches use of a conductive grid coated with a photoconductive material, and a conductive control grid, in which an electrostatic latent image is formed on the conductive grid, and different electric fields are formed between the conductive grid and control grid so as to modulate flow of the corona ions for forming an image on the recording member. In U.S. Pat. No. 3,645,614, the screen comprises an insulating material overlaid with a conductive material, and the insulating material comprises a photoconductive material. An electric field to prevent the ion flow from passing through the screen is formed at the openings or perforations for permitting the ion flow to pass therethrough owing to the electrostatic latent image formed on the screen. U.S. Pat. No. 3,713,734 teaches use of a four-layer screen consisting of a photoconductive substance, a first conductive substance, an insulating substance, and a second conductive substance, in which an electrostatic latent image is formed on the photoconductive substance in conformity to the original picture image by the processes of electric charging and image exposure. Also, in the case of forming an image on the recording member by modulating the flow of the corona ions through the electrostatic latent image, the second conductive substance of the screen is imparted a voltage having a polarity opposite to that of the electrostatic latent image on the screen, since the image is in a single polarity. U.S. Pat. No. 3,647,291 teaches the formation of an electrostatic latent images of mutually different polarities on a two-layer screen consisting of a conductive substance and a photoconductive substance in correspondence to a bright image portion and a dark image portion so as to modulate passage of the corona ion flow by the latent image formed on the screen.
As described, the screen is capable of forming an electrostatic latent image on a recording member without the latter being contacted by the former, and greatly differs from the conventional electrophotographic process using the TESI method. In the electrophotography using the screen, however, there is a tolerance for the distance defined between the screen and the recording member if such tolerance is exceeded, the latent image on the recording member would be blurred or unnaturally thick with a result that a visible image faithful to the original image could not be provided. Further, the method whereby ion flow is modulated by an electrostatic latent image preformed on the screen is not preferable in that the latent image on the screen may happen to be erased. On the other hand, there is also a tolerance for the intensity of the field acting between the screen and the recording member during modulation of ion flow and whenever such tolerance is exceeded, there will be obtained no good result.